Since 2000, Mobile, Ala., has undergone a major project to protect its buildings and equipment from persistent lightning strikes. The Mobile Board of Water and Sewer Commissioners (also known as Mobile Area Water and Sewer System, MAWSS) has been installing lightning protection systems with the help of McCrory & Williams Inc., and has developed a standard for use in future projects.
The standard was officially published in July 2007 on the board's Web site (mawss.com). At the request of the board, McCrory & Williams prepared the specifications, which were approved by the board and will now be used as a standard for all future lightning protection projects.
The standard is supplemental to two nationally recognized industry standards for lightning protection: Underwriters Laboratories (UL) Standards for Safety, Lightning Protection Components, UL 96 A and the National Fire Protection Agency (NFPA) 780 Standards for the installation of Lightning Protection Systems. The installer for lightning protection systems should follow the UL and NFPA standards and should provide proof of an Underwriters Laboratories Master Label certificate.
A complete lightning protection system is threefold: It should include the use of lightning rods on building and facility rooftops; surge suppressors on all incoming lines for power, telephone, and computer lines; and proper grounding. So when budget time comes around, use the following information to your advantage when lobbying for this equipment:ROD USAGE
The industry typically uses three types of lightning rods:
- Lightning arrestors, which are cylindrical rods (3/8 - to ½-inch in diameter and 10 to 18 inches long) with a point on the upper end.
- Lightning dissipaters, which have as many as 500 dissipation electrical wires in the upper end.
- Lightning attractors, which are 10 to 50 feet tall and are frequently mounted separately and away from a building. They include electronics that stimulate magnetic fields and therefore require a power source.
When installing the rods, workers should understand that the lightning protection system for each structure should be designed to provide a Faraday Cage (an enclosure that blocks out external electrical fields) around the structure to delay or dissipate the formation of lightning strikes.
Installers should mount individual stream-delaying air terminals on the top of the structure on all outside corners at 20-foot intervals around the perimeter and at 50-foot intervals across the top. Each air terminal must be provided with two electrically conductive paths to ground. A common wire that extends down the side of the building and is attached to a ground rod at intervals of no more than 100 feet should bond all lightning rods.
All equipment that protrudes above the roof line, including air conditioners, air handlers, vents, air conditioning ducts, aerator mixer motors, and relief valves, must have its own air terminals that are properly bonded to the ground rods for that building. Air terminals should be 18 inches higher than the equipment.
Steel parts of electrically conductive structures such as light poles, towers, and steel superstructures should not be used because they corrode or rust over time and therefore lose their ability to provide electrical continuity to adjacent parts. That is especially true in coastal areas and inside sewer plants where corrosive gases are present.
Also discouraged is the use of gutters or metal down spouts for conductors between air terminals and ground rods. In both cases it is better to utilize a separate continuous piece of wire.